Twelve soldiers and a civilian have been arrested so far in connection with last month's failed coup in Niger, Defence Minister Mahamadou Karidjo said on Wednesday.

"Investigators have established the involvement of 13 people, including 10 soldiers from the Niger Armed Forces (FAN), two National Guard members and a civilian," the minister said on state television, which said the preliminary stage of the inquiry had finished.

He said four of the suspects had "admitted they were preparing a coup d'etat" on December 18, 2015 when the former French colony was to mark its 57th anniversary.

"The officers whose guilt has been established... will appear before a military tribunal," he said.

He said another 10 civilians and three customs agents had also been detained while investigators check "their possible implication" in the planned putsch, Karidjo said. He refused to identify any of the detainees.

In mid-December, Niger's interior minister had named several senior figures arrested in connection with the plot, among them former chief of army staff General Souleymane Salou and two top officers with elite anti-terror units.

Karidjo also said investigators had spoken to jailed opposition figure Hama Amadou about the failed coup. Amadou, a contender in next month's presidential election, is currently serving time for his alleged role in a baby trafficking scandal.

When the government announced it had foiled an alleged coup, opposition figures quickly rubbished the claim, with Amadou Boubacar Cisse, also a candidate for upcoming presidential polls, saying there was "no proof". He also accused the government of seeking to manipulate the political climate ahead of the elections.

Tensions have increased in Niger in the run up to the February 21 presidential election when five candidates are running for the top job, including Incumbent Mahamadou Issoufou who was elected in 2011.

At the end of December, the opposition said many of its members had been arrested for unknown reasons in a move it claimed was "directly connected to the president".

A protest planned for Sunday to denounce the "arbitrary arrests" was banned.

And on Monday, lawyers across the west African country staged a one-day strike over the authorities' refusal to let them assist clients detained in connection with the alleged coup plot.